Typically, conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram, with for example Zernike polynomials, indicates the presence of aberrations.
The reference wave of an interferometer should be “perfect”; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optics through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always includes some artifacts from the optical system through the reference wave passes. While these artifacts can in theory be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly clean interferogram.
To address this problem, an interferometer of relatively simple design, the “point diffraction interferometer”, has been developed. These devices generate reference waves through a point (a pinhole) in the path the test beam. The light diffracting around the occluding disk or through the pinhole, unencumbered by beam splitters, mirrors, lens, or other optical elements, closely approximates the ideal spherical wave desired for a reference wave.
Furthermore, it is also desirable for such interferometers and interferometric instruments to produce high contrast interferograms which enable precision measurements. To this end it is desirable to utilize a “perfect” reference wave; that is it should be simple and well characterized, such as a spherical wave. Perfect reference waves are typically produced by incident-light diffraction at a small, central pinhole, or around an occluding disk.
Additionally, it is also desirable to ensure precise control of the phase shifting process for even greater measurement precision and accuracy. Accurate phase shifting, however, is not easily accomplished. In particular, the common phase shifting method of translating mirrors in a direction of the optical path is difficult due to the exaggerated effects of even minute mirror motions. Various other methods and arrangements have been developed to facilitate and improve phase shifting. For example, a Wollaston prism and a quarter wave plate may be used to shear light into two beams. A wedge glass is then used to vary the optical path difference of one of the beams relative to the other to thereby impart a phase shift.
One difficulty with the point diffraction interferometer is that phase shifting between the reference and test waves is not easily accomplished. Unfortunately, phase shifting is often necessary to improve the accuracy of the instrument.
Thus, there is a need in the art for an improved low light level pulsed phase measurement system.